The adsorption of CO on W(110) surfaces was studied using thermal desorption spectroscopy (TDS), and core and valence level spectroscopy. At 120 K, CO forms a tilted structure at lower coverages ($alpha$ 1), whereas it adsorbs normal to the surface at higher coverages ($alpha$ 2). Tilted structures have been suggested to be precursors of dissociative chemisorption; however, experimental evidence is provided for the non-dissociative chemisorption of CO at temperatures above 900 K (which is referred to as the $eta$ -state): TDS shows first order desorption kinetics. The core and valence level spectra of O/W(110) and those of $eta$ -CO/W(110) are different. Most importantly, the 4$sigma$ molecular orbital of CO can be identified in the valence level spectra of the $eta$ -CO.